Offline the disk if you can by going to Storage > Volumes > Select your pool (e.g. tank) > Volume Status (on the status bar at the bottom). I could not do this since the disk was already unavailable and hence offline

NOTE the disk's name, like ada5xx

Next, navigate to the Storage > View Disks and jot down the disk's serial number. You will need this to physically identify the disk later.

You can watch the pool rebuilding (resilvering) either on the GUI or on the CLI:

[root@freenas] ~# zpool status tank
pool: tank
state: ONLINE
status: One or more devices is currently being resilvered. The pool will
continue to function, possibly in a degraded state.
action: Wait for the resilver to complete.
scan: resilver in progress since Wed Apr 19 21:42:55 2017
49.1G scanned out of 5.89T at 301M/s, 5h39m to go
9.79G resilvered, 0.81% done
config: